1. Field of the Invention
This invention relates in general to concentric tube heat exchangers. More particularly, it relates to an improved recuperator for recovering exhaust heat from a Brayton Cycle gas turbine engine, Ericsson Cycle engine, or similar recuperated engine.
2. Description of Prior Art
The thermodynamic efficiency and resulting fuel economy of a gas turbine (Brayton Cycle) engine can be greatly increased by using an exhaust gas heat exchanger to recover heat from the low pressure exhaust stream to preheat the high pressure air between the compressor and combustor. The heat thus recovered in the preheating process, which would otherwise be wasted in the exhaust, does not have to be supplied by the combustor. As a result, the cycle efficiency is typically doubled from about 15% without a heat exchanger to 30% with a heat exchanger. Newer types of engines, such as the Afterburning Ericsson Cycle of my U.S. Pat. No. 5,894,729 (1999), make even better use of an exhaust gas heat exchanger and can achieve cycle efficiencies of over 40%.
There are two types of exhaust gas heat exchangers: recuperators and regenerators. Although the names are frequently used interchangeably, a recuperator usually refers to a heat exchanger where the high pressure compressor flow and the low pressure exhaust flow are continuously separated by walls and the heat transfer takes place through those walls. A regenerator usually refers to a heat exchanger where the same walls are alternately exposed to the high and low pressure flows. Although a regenerator is usually smaller than a comparable recuperator, the seals and moving parts needed for the flow switching causes mechanical complexity, flow leakage, lower heat recovery and higher cost. Therefore, recuperators are becoming the preferred type of exhaust gas heat exchanger.
A recuperator has requirements that are unique from other types of heat exchangers. First, it must be able to capture the maximum percentage of the available exhaust heat (have high effectiveness). The higher the effectiveness, the more efficient the engine becomes. However, high effectiveness generally requires more pressure drop in both the high pressure compressor outlet flow and the low pressure exhaust flow. The flow work represented by these pressure drops reduces the engine efficiency and can offset the gain from a higher effectiveness. Therefore, the pressure drop through the recuperator must be maintained as low as possible while still obtaining the highest thermal effectiveness. In addition, the recuperator should be easily and economically fabricated, be able to withstand the pressure load from the high pressure flow, allow for thermal growth during heating and cooling transients, be tolerant of fouling from exhaust products, and be capable of withstanding high exhaust temperatures.
Prior art recuperators have compromised one or more of those requirements. The most common approach has been to use a plate-fin heat exchanger. This type of recuperator is generally made of flat sheets interleaved with corrugated sheets that are furnace brazed or welded together. A leading prior art plate-fin heat exchanger is documented in U.S. Pat. No. 5,983,992 (1999). This type of recuperator can attain fairly high effectiveness but is expensive to make because of the large number of parts, many of which are thin wall plates subject to damage during manufacture. Furthermore, since recuperators operate at temperatures where creep strength is low, the pressure loads from the high pressure side can cause the thin plates to distort and shorten the life of the recuperator. Finally, the large number of highly stressed welds increases manufacturing cost and provides potential locations for failure and leakage.
The U.S. Army M1A1 main battle tank is powered by a gas turbine having an annular plate recuperator and is described in U.S. Pat. No. 5,388,398 (1995). This recuperator consists of many annular plates that are also very complex to manufacture and maintain leak free.
The spiral recuperator has been developed in an effort to avoid the complexity of plate type heat exchangers while also using the curved surfaces to hold pressure with less material stress. U.S. Pat. No. 4,883,117 (1989) proposes a typical spiral type recuperator. Spiral recuperators have a significant problem with thermal xe2x80x9cshort circuitingxe2x80x9d that prevents them from achieving high thermal effectiveness. The spiral path puts cold fluid and warm fluid in the same flow in direct or close contact and causes the flow to have a driving potential to become the same temperature. Since the objective is to obtain the maximum temperature difference between the inlet and outlet, the xe2x80x9cshort circuitxe2x80x9d effect can greatly reduce the thermal effectiveness.
A recent advance in spiral recuperators is the Rolls-Royce heat exchanger of U.S. Pat. No. 6,115,919 (2000). Although of spiral construction, both the high and low pressure flows are true counterflow and run along the axis of the spiral with no possibility for xe2x80x9cshort circuitsxe2x80x9d. The recuperator is intended for mass production by being formed of continuous strips that are rolled in a spiral to form the recuperator. Nevertheless, the recuperator is still quite complex and requires many welds of thin material at the header holes and sheet edges. The manufacturing cost will remain high due to the number of complex welds and the need to accurately align the spiral sheets. As with the plate-fin heat exchanger, the weld joints are always a potential location for failure and leakage.
The previously described recuperators make use of corrugated surfaces, wavy surfaces, fins or similar devices to increase turbulence and heat transfer. These devices are indeed effective in raising effectiveness, but they also increase pressure drop. It has been common practice in compact heat exchanger design to operate in the turbulent flow range and to avoid laminar flow. However, with small hydraulic diameters, significant heat transfer coefficients can be achieved with laminar flow. Just as important, with laminar flow, the overall heat transfer rate can be increased while the pressure drop is decreased. This is because the heat transfer coefficient in laminar flow is dependent only on passage geometry and not on the flow velocity. Additional flow paths can be added to proportionately increase the overall heat transfer conductance while at the same time proportionately decreasing the pressure drop. This characteristic is exactly what is needed for a high performance recuperator.
U.S. Pat. No. 5,725,051 (1998) describes a heat exchanger that is successfully used as a laminar flow recuperator. Although not currently used as an engine recuperator, a plastic version used in home ventilation systems achieves 94% effectiveness with a pressure drop of only 0.16 inch of water. The recuperator allows fresh outside air to be exchanged with inside air with only a 6% of the un-recuperated air conditioning or heating load. The disadvantage of this heat exchanger is that it has a very complex header system to distribute the two streams to their respective heat exchange flow paths.
Concentric tube heat exchangers are used frequently in applications other than recuperators. Although possessing the advantage of being able to use simple tubular construction, prior art concentric tube heat exchangers have had several limitations for use as recuperators. U.S. Pat. No. 6,012,514 (2000) is a simple concentric tube heat exchanger that is easy to manufacture and maintain. However, it uses gasketed construction that is not suitable for the high temperatures and pressures of a recuperator. More importantly, like other concentric tube heat exchangers such as in U.S. Pat. No. 4,204,573 (1980), U.S. Pat. No. 4,254,826 (1981), and U.S. Pat. No. 4,440,217 (1984), only two tubes are used in the concentric tube assemblies. With this arrangement, one flow passage is within the circular passage of the center tube and the other is in the annular region between the center and outer tube. It is difficult to achieve high rates of heat transfer into the center tube, particularly if attempting to achieve low pressure drop by using laminar flow.
It is the primary aim of this invention to overcome the disadvantages of current engine exhaust gas recuperators discussed above and to achieve high thermal effectiveness, low pressure drop, long life, and economy of manufacture by implementing the several objects listed below.
It is an object of this invention to provide a recuperator attaining a minimum of 90% effectiveness with reasonable size and cost.
To attain the high effectiveness it is an object to have linear, counterflow, flow paths to prevent any potential for thermal xe2x80x9cshort circuitsxe2x80x9d.
To attain the high effectiveness it is an object to minimize the losses due to axial conduction in the recuperator.
To attain the high effectiveness it is an object to define methods to account for, minimize and accommodate effectiveness loss due to flow misdistribution and manufacturing tolerances.
To attain the high effectiveness it is an object to have an easily insulated recuperator to minimize ambient heat loss.
It is another object to minimize the pressure drop through both the high pressure and low pressure sides of the recuperator.
It is also an object to have sufficient margin to accommodate exhaust gas fouling of the low pressure flow passages.
It is a further object to be able to use lower cost materials at their upper limits of strength and creep resistance by using only cylindrical or stayed surfaces to minimize material stress.
It is a yet a further object to avoid inducing thermal gradient stresses by having all non-isothermal portions of the recuperator able to freely expand and contract.
It is still another object to define a method of construction that requires no special tooling or manufacturing processes.
It is another object to minimize the construction cost by being able to use commercially available tubing materials as the primary heat transfer passages.
To implement the stated objects of the invention, an annular flow, concentric tube, counterflow recuperator has been devised and a novel method of manufacturing the recuperator has been developed. The principal feature of the concentric tube recuperator is that it allows a high performance (high temperature, high effectiveness, low pressure drop) recuperator to be made by simply welding, brazing, or otherwise joining standard commercial tubing with no special tooling. The only parts of the recuperator that are not made from commercial tubing are the header plates, concentric tube assembly spacers, and outside insulation. The header plates can be made by any competent machine shop with no special tooling by simply boring holes in circular plates. The holes do not even have to be accurately located so long as they are concentric between plates with reasonable accuracy. The concentric tube assembly spacers are simple annular pieces that can be stamped, extruded, or made on a lathe. The insulation blankets are also simple to manufacture.
The basic element of the recuperator is a concentric tube assembly that, in the preferred embodiment, is comprised of four concentric tubes that enclose three concentric annular flow passages. The low pressure exhaust flows through the inner and outer annular passages while the high pressure compressor exit air flows through the annular passage that is between the two low pressure passages. The high and low pressure flows are in opposite directions to achieve the high effectiveness that is only available with a counterflow heat exchanger. Heat is transferred from the exhaust gas to the compressor air though the tube walls on each side of the high pressure passage. Two low pressure passages are provided for each high pressure air passage to compensate for the lower pressure (and therefore lower density) of the exhaust gas. The 2/1 flow passage ratio allows close tube gaps to be used to maximize heat transfer while providing a larger low pressure flow area to minimize the pressure drop of the low density exhaust.
Multiple concentric tube assemblies are used to make a recuperator. The tube assemblies terminate in header assemblies located at each end of the concentric tube assemblies. The headers are made of simple plates and rings that serve the dual function of structurally locating the concentric tube assemblies and directing the flow to the proper passage in the concentric tube assemblies. High and low pressure flow tubes provide flow passages connecting the recuperator to the engine compressor air and exhaust tubing respectively.
In the preferred embodiment, the high heat transfer and low pressure drop is accomplished by sizing the number of concentric tube assemblies and the diameters of the tubes in the assemblies to allow the recuperator to operate with laminar flow in both the high and low pressure passages. Operating in the laminar region provides two advantages. First, standard, low cost, commercial tubing can be used to build the concentric tube assemblies. No additional heat transfer enhancement devices such as ribs, fins, spiral wires, or such devices are necessary to promote turbulence. High heat transfer rates are obtained using smooth tubes, spaced closely together. Second, in laminar flow, the heat transfer coefficient is defined solely by the tube spacing within the individual concentric tube assemblies. Additional concentric tube assemblies can then be added in parallel to proportionally increase the heat transfer area (and therefore proportionally increase overall heat transfer rate) while simultaneously proportionally reducing the pressure drop. The laminar flow characteristic of proportionally increasing heat transfer rate while proportionally decreasing pressure drop is the key to meeting the objective of high effectiveness and low pressure drop.
The objective of defining a method of construction that requires no special tooling or manufacturing processes is met by having two types of concentric flow assemblies, a basic concentric tube assembly and a xe2x80x9ctoolingxe2x80x9d concentric flow assembly. The invention provides a method of manufacturing a laminar flow concentric tube recuperator comprising the steps of:
(a) building at least two tooling concentric flow assemblies by spacing the four tubes with annular spacers to form a rigid assembly.
(b) manufacturing four header plates for each end such that each plate has a number of holes corresponding to the number of concentric tube assemblies, with the hole diameters on each plate corresponding to the outside diameter of a corresponding tube in the concentric tube assembly, and concentric locations of the corresponding holes in all four plates.
(c) attaching a header plate with the largest holes to each of the tooling concentric flow assemblies.
(d) attaching the largest tubes of the basic concentric tube assemblies to the header plates from step (c).
(e) attaching a header ring to each of the header plates from step (c).
(f) attaching a header plate with the second largest holes to each of the tooling concentric flow assemblies and to the header rings from step (e).
(g) attaching the second largest tubes of the basic concentric tube assemblies to the header plates from step (f).
(h) attaching a header ring to each of the header plates from step (f) and to the header ring of step (e)
(i) repeating the process until the metal portions of the recuperator are completed.
(j) wrapping the recuperator metal portions with an insulation blanket.